The invention relates to an LED driver circuit comprising at least one string of LEDs connected in series, and a power supply for converting a mains voltage into an output voltage to be applied to said at least one string of LEDs.
More particularly, the invention relates to high power lighting applications such as industrial lamps, sport field lamps, street lamps and the like, wherein an array of a plurality of LEDs is powered by a common power supply.
Since the forward voltage of a single LED, typically in the order of magnitude of 1 to 5 V, is significantly smaller than the mains voltage of, e.g., 400 VAC, 230 VAC or 110 VAC, it is necessary to convert the mains voltage into an output voltage that is suitable for the LEDs. When a plurality of LEDs are connected in series, the output voltage should correspond to the sum of the forward voltages of the LEDs in the string.
Most conventional LED driver circuits comprise a plurality of strings which each have only a relatively small number of LEDs, so that the output voltage will be lower than the mains voltage. However, when a plurality of strings are connected in parallel to a common power supply, the output current must be relatively high, which leads into increased system losses, and additional measures must be taken to assure a correct current balance between the parallel LED strings. In general for each LED string a separate converter operated in a current mode is applied to regulate the LED current. In addition, these systems require numerous connections and interconnection wires, so that the costs for the electronic components and their installation are relatively high.
EP 2 315 497 A1 and EP 2 458 940 A1 describe LED driver circuits which have a two-stage power supply. The first stage is a converter with a power factor correction function which converts the AC mains voltage into a DC voltage and assures compliance with the AC grid regulations. The second stage is a driver that regulates the current in the LED string or strings.